1. Field
This disclosure relates to processing of audio-frequency signals.
2. Background
Active noise cancellation (ANC, also called active noise reduction) is a technology that actively reduces ambient acoustic noise by generating a waveform that is an inverse form of the noise wave (e.g., having the same level and an inverted phase), also called an “antiphase” or “anti-noise” waveform. An ANC system generally uses one or more microphones to pick up an external noise reference signal, generates an anti-noise waveform from the noise reference signal, and reproduces the anti-noise waveform through one or more loudspeakers. This anti-noise waveform interferes destructively with the original noise wave to reduce the level of the noise that reaches the ear of the user.
An ANC system may include a shell that surrounds the user's ear or an earbud that is inserted into the user's ear canal. Devices that perform ANC typically enclose the user's ear (e.g., a closed-ear headphone) or include an earbud that fits within the user's ear canal (e.g., a wireless headset, such as a Bluetooth™ headset). In headphones for communications applications, the equipment may include a microphone and a loudspeaker, where the microphone is used to capture the user's voice for transmission and the loudspeaker is used to reproduce the received signal. In such case, the microphone may be mounted on a boom and the loudspeaker may be mounted in an earcup or earplug.
Active noise cancellation techniques may be applied to sound reproduction devices, such as headphones, and personal communications devices, such as cellular telephones, to reduce acoustic noise from the surrounding environment. In such applications, the use of an ANC technique may reduce the level of background noise that reaches the ear (e.g., by up to twenty decibels) while delivering useful sound signals, such as music and far-end voices.